Technical Field
This disclosure relates in general to solid state thermoelectric devices, in particular to thermoelectric generators (TEGs) amenable to be fabricated with planar processing technologies and related techniques of heterogeneous or hybrid 3D integration.
Discussion of Related Prior Art
Thermoelectric generators (TEGs) are earnestly investigated as low enthalpy waste heat exploitation devices of outstanding robustness, reliability and virtually unlimited service life, besides being made of environmental friendly materials.
As power consumption of increasingly popular electronic devices is constantly minimized, TEGs begin to be considered as supplementary power source in conjunction or even in substitution of batteries or other energy storage devices like super-capacitors.
There is an increasing number of publications concerning thin film technology TEGs exploiting well established processing techniques developed in the Microelectronics and Micro-Electro-Mechanical-Systems (MEMSs), like planar processing, micromachining implant and post implant treatments, flip-chip and bonding techniques and alike.
The doctorate thesis “Silicon-Micromachined Thermoelectric Generators for Power Generation from hot gas streams” by Israel Boniche, University of Florida, 2010, and “Monolithic integration of VLS silicon nanowires into planar thermoelectric generators” by Diana Davila Pineda, Autonomous University of Barcelona, 2011, offer an extensive introductory review of state-of-the-art practices in the field of thermoelectric devices for solid state heat pumps and power generators.
The review encompasses also two families of TEGs manufactured with silicon-compatible micro and nano technologies: in devices of a first family heat flow is parallel and in the other family orthogonally to the substrate. The architectures of these integrated TEGs generally comprise a number of unit cells having n-p doped legs, arranged in such a way that the unit cells are thermally in parallel and electrically in series.
Typically, integrated TEG devices in which heath flows parallel to the substrate may have conductive legs of thermoelectrically active materials deposited over a very high thermal resistance material or a membrane, suspended several hundreds of micrometers above the substrate, or the legs of active materials themselves are free-standing [membrane-less].                1. Other relevant examples are reported in:        2. Huesgen, T.; Wois, P.; Kockmann, N Design and fabrication of MEMS thermoelectric generators with high temperature efficiency. Sens. Actuators A 2008, 145-146, 423-429.        3. Xie, J.; Lee, C.; Feng, H. Design, fabrication and characterization of CMOS MEMS-based thermoelectric power generators. J. Micromech. Syst. 2010, 19, 317-324.        4. Wang, Z.; Leonov, V.; Fiorini, P.; van Hoof, C. Realization of a wearable miniaturized thermoelectric generator for human body applications. Sens. Actuators A 2009, 156, 95-102.        5. Wang, Z.; Fiorini, P.; Leonov, V.; van Hoof, C. Characterization and optimization of polycrystalline Si70% Ge30% for surface micromachined thermopiles in human body applications. J. Micromech. Microeng. 2009, doi: 10.1088/0960-1317/19/9/094011.        6. Su, J.; Leonov, V.; Goedbloed, M.; van Andel, Y.; de Nooijer, M. C.; Elfrink, R.; Wang, Z.; Vullers, R. J. A batch process micromachined thermoelectric energy harvester: Fabrication and characterization. J. Micromech. Microeng. 2010, doi: 10.1088/0960-1317/20/10/104005.        7. Yang, S. M.; Lee, T.; Jeng, C. A. Development of a thermoelectric energy harvester with thermal isolation cavity by standard CMOS process. Sens. Actuators A 2009, 153, 244-250.        8. Kao, P.-H.; Shih, P.-J.; Dai, C.-L.; Liu, M.-C. Fabrication and characterization of CMOS-MEMS thermoelectric micro generators. Sensors 2010, 10, 1315-1325.        9. Wang, Z.; van Andel, Y.; Jambunathan, M.; Leonov, V.; Elfrink, R.; Vullers, J. M. Characterization of a bulk-micromachined membraneless in-plane thermopile. J. Electron. Mater. 2011, 40, 499-503.13.        10. U.S. Pat. No. 7,875,791 B1 “Method for manufacturing a thermopile on a membrane and a membrane-less thermopile, the thermopile thus obtained and a thermoelectric generator comprising such thermopiles” Vladimir Leonov, Paolo Fiorini, Chris Van Hoof (2011)        11. Miniaturized thermopile on a membrane are also described by A. Jacquot, W. L Liu, G. Chen, J. P Flrial, A. Dauscher, B. Lenoir, in “Fabrication and Modeling of an in-plane thermoelectric micro-generator”, Proceedings ICT'02. 21st International Conference on Thermoelectrics, p. 561-564 (2002).        
Other examples of parallel heat flow TEG structures rely on the ability of growing or defining populations of parallel and extremely slender conductors (nanowires) with a mean diameter of few tens of nanometers on a planar substrate of low heat conductivity and in eventually stacking tile-modules to form a thermo-electrical active septum, through which heat flows in the same direction of the parallel nanowires. The articles: “A. I. Hochbaum, R. K. Chen, R. D. Delgado, W. J. Liang, E. C. Garnett, M. Najarian, A. Majumdar, and P. D. Yang, Nature 451, 163-U5 (2008)” and “A. I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.-K. Yu, W. A. Goddard Iii, and J. R. Heath, Nature 451, 168-171 (2008)”; WO2009/125317; EP1,083,610; WO2011/007241; WO2011/073142; offer a review of practices following such an approach.
U.S. Pat. No. 7,875,791 B1 (by Leonov et al.) discloses thermopiles that may be supported by a membrane layer or that may be self-supporting. Despite the apparent easy manufacturability of these devices, heat is forced to move in a complicated structure with significant thermal losses. In addition, in some cases adhesive are needed in order to assure thermal contact to a heat source at the top or the bottom surface of the initial substrate. This results in poor thermal coupling at system level, lossy thermal paths and mechanical fragilities that penalize performance of the thermopile.
A second family of TEG devices is often referred to as “out-of-plane” heat flux TEGs. They are characterized by the fact that heat flows orthogonally to the substrate. In these devices the thermoelectrically active materials are usually laid on or are part of high aspect-ratio supporting structures standing onto the substrate. Despite a more sophisticated and apparently expensive fabrication process, this configuration minimizes thermal losses, simplifies thermal coupling at system level enhancing overall performance.
Being manufactured by conventional CMOS\BiCMOS\MEMs materials and processes, the “out-of-plane” heat flux TEGs are amenable to miniaturization and integration in microelectronic and optoelectronic devices, among other applications.
Examples are reported by M. Strasser et al. in “Miniaturized Thermoelectric Generators Based on Poly-Si and Poly-SiGe Surface Micromachining”, (presented in The 11th International Conference on Solid-State Sensors and Actuators, Munich, Germany, Jun. 10-14, 2001) and “Micromachined CMOS Thermoelectric Generators as On-Chip Power Supply” (presented in The 12th International Conference on Solid-State Sensors and Actuators and Microsystems, Boston, USA, Jun. 8-12, 2003).
Out-of-plane or orthogonal heat flux thin film structures are useful for innumerable applications, for example for micro power generation or for temperature management in complex integrated systems, for energy recovery or harvesting.
Electric power yield from a given heat flow and electric power yield versus the footprint area of out-of-plane or orthogonal heat flux devices of the prior art, based on a common semiconductor or any material compatible with ICs fabrication processes, are yet poor and there is a need of more efficient and power intensive devices.